... > 0 G x0r B G (x0, r)B G (x0, r) = {y ∈ X : G( x0, y, y) < r}. G τ (G) Xτ (G) G G (X, G) G G(x, y, y) = G( y, x, x) x, y ∈ X G (X, G) (X, d G ) d G d G (x, y) = G( x, y, y) +G( y, x, ... y, z) ≤ G( x, a, a) + G( a, y, z) x, y, z, a ∈ XX G G G X = G : X ì X × X → R+ G( x, y, z) =0 x = y = z,1 .(X, G) G G G 1 G 2 G 3 G 4 G G5x, y, z, a ∈ Xx = y = z G( x, y, z) = 0 G G(x, y, ... z))= G( x, a, a) + G( a, y, z)(X, G) G (X, G) (X, G ) G f : (X, G) → (X, G ) f G a ∈ Xε > 0 δ > 0 x, y ∈ X G( a, x, y) < δ G( f(a), f(x), f(y)) < ε f G X f G a ∈ X(X, G) G {xn}X...